Motorized operators for quarter turn or butterfly valves and other partial turn valves (e.g. ball or plug) typically include both a reversible, motorized actuator and a partial turn operator. The motorized actuator typically rotates a geared down shaft which drives the partial turn operator. The partial turn operator also typically drives the valve shaft through a reduction gear arrangement.
Motorized operation is typically controlled by a pair of adjustable position limit switches. However, torque switches are also typically provided for backup control. The torque switches shut off or stop the operation of the motor when the torque in the actuator is sufficient to move the actuator spring pack a preset distance necessary to trip either switch. The torque and position limit switches are typically associated with the motor/actuator.
Motor operated, quarter turn butterfly valves are commonly used in nuclear and other power plants to perform routine opening and closing of water and steam lines and, more importantly, for those valves designated as safety related, to perform such opening or closing to assure the prompt, safe shutdown of the plant under accident or other emergency or abnormal operating conditions. For those valves of this class whose safety function is to open or close a water, steam or other line, it is important that the torque switches be properly adjusted to the particular set of plant operating conditions which may be encountered. If a torque switch setting is too low, it may impede proper operation by shutting off power to the motor before full opening or closing of the valve is achieved. If the torque switch setting is too high, it may not prevent the valve or actuator from being damaged in the event of excessive torque due to valve malfunction or an obstruction to valve operation.
Torque switches are set according to specific valve design specifications including type of valve, and specific operating conditions such as temperature, flow, and pressure. The manufacturer's recommended open and close torque settings are sometimes displayed on the torque switch calibration tag on the actuator. Torque switches are typically provided with dials bearing incremental, numbered markings (e.g., 1-5). However, these markings do not correspond with absolute torque values.
As the actual numerical setting of the torque switch is an arbitrary number, and the relation of one switch setting to another can vary significantly, torque switches could be set to specific torque values only by bench testing before installation or after removal. In-plant torque switch adjustments have typically consisted of verifying that full valve closure is achieved prior to close torque switch cut off. If this does not happen, implying that the close torque switch has been set too low, a small adjustment is made and the procedure is repeated. After successful completion of this procedure, the only information that is definitely known about the torque switch settings is that they are sufficiently high enough not to impede the nominal valve closure.
It would be desirable to determine the actual torque output at the operator for both the opening and closing torque switch operation during on-site testing of a fully assembled and on-line motorized valve operator. It would be desirable to achieve this by applying a non-intrusive artificial load to the valve operator, which accurately represents an applied valve torque load, and measuring that load as it is driven to torque switch trip. It is usually not possible to achieve sufficient torque load to trip the torque switches simply by closing the valve against its seat in the no-flow condition. This is because the seats are typically elastomeric, and would be damaged for that amount of torque. The maximum torque levels on the full flow condition are developed before full closure, and these are typically less than the torque required for torque switch trip. In obtaining the actual numerical torque values for torque switch settings, a better assessment of initial settings, as well as performance changes due to wear, aging or maintenance activities can be accurately accounted for.